CN112191259B - MXene/Au photocatalytic nitrogen fixation material, and preparation method and application thereof - Google Patents

Abstract

The invention discloses an MXene/Au photocatalytic nitrogen fixation material, a preparation method and application thereof, wherein the preparation method of the MXene/Au photocatalytic nitrogen fixation material comprises the following steps: (1) preparation of layered Ti ₃ C ₂ Materials for standby; (2) weighing layered Ti ₃ C ₂ A material containing 5-10% by volume of N in hydrogen ₂ /H ₂ Annealing treatment is carried out for 5-7 hours at the temperature of 180-220 ℃ in mixed atmosphere to obtain the r-Ti ₃ C ₂ (ii) a (3) Form a layer of Ti ₃ C ₂ Or r-Ti ₃ C ₂ The material is ultrasonically and uniformly dispersed in water, then an Au ball solution coated by sodium citrate or a gold ball solution coated by CTAB is added, the material is ultrasonically and uniformly dispersed, a solvent is evaporated in a rotary manner, and the material is dried to obtain the material. The invention improves the photocatalytic nitrogen fixation efficiency of the material by means of rotary evaporation, a solvent dynamics driving method and a surface charge repulsion/adsorption principle and by means of the local surface plasmon resonance effect of the Au ball.

Description

A kind of MXene/Au photocatalytic nitrogen fixation material, its preparation method and application

technical field

The invention belongs to the field of photocatalytic materials, and in particular relates to an MXene/Au photocatalytic nitrogen fixation material, a preparation method and application thereof.

Background technique

Energy is the most basic driving force for the sustainable development of human society, and it is also an important material basis for human survival. Traditional fossil fuels still occupy the dominant position in today's energy supply and demand system. However, with the increasing depletion of fossil energy, the sustainable development of the world economy will be severely restricted, and the energy problem has become one of the main problems facing human beings. Vigorously developing and seeking green and renewable new energy has become the primary task of research all over the world. As a typical green renewable clean energy, ammonia is not only a vital chemical raw material for the manufacture of fertilizers, explosives, medicine, plastics and other industries, but also an important energy storage intermediate and carbon-free energy carrier. More importantly, ammonia provides a completely new energy market that is not limited by geographical distribution, bringing unique market advantages and opportunities to it. Therefore, ammonia has become one of the frontier research directions in the field of green and sustainable clean energy, which solves the market concentration caused by geographical factors and a series of potential problems caused by it. Develop a sustainable dependence on the transition to fossil fuels.

At present, the industrial production of _NH3 still adopts the traditional Haber-Bosch process technology, which uses iron or ruthenium-based materials as catalysts, using high-purity _N2 and _H2 gas streams at high temperature (300-550 °C) and It is realized under high pressure (20-30 MPa). The NH ₃ production process is a process of high energy consumption, the preparation conditions are relatively harsh, and the requirements for production equipment are relatively high, but it still dominates the current industrial ammonia synthesis industry. Therefore, exploring the realization of high-efficiency reaction of nitrogen and water to synthesize ammonia at room temperature and pressure, and at the same time using sustainable green and clean energy as the driving energy supply in the process of ammonia synthesis, will be able to completely overcome the traditional Haber-Bosch process technology for ammonia synthesis. The problems of high energy consumption, high carbon emissions and safety, etc., have important practical research significance for alleviating the energy crisis and meeting the growing demand for ammonia, which will contribute to the sustainable development of human society and economy as well as human production, Lifestyle brings tremendous change and economic benefits. Photocatalytic nitrogen fixation uses solar energy as an energy driving force to achieve zero carbon emission in chemical industrial production in the true sense. It is a clean, green, sustainable, high-efficiency, and high-selectivity ammonia synthesis process technology.

The design and construction of photocatalyst materials is the leading and core of photocatalytic ammonia synthesis technology. Compared with traditional bulk nanomaterials, 2D semiconductor nanosheets can effectively improve electron mobility and surface energy, thereby ensuring effective light absorption utilization and adsorption of target reactants, thereby promoting the occurrence of interfacial catalytic reactions, and at the same time having It is conducive to the rapid migration of photogenerated electrons from the interior of the material to the surface, ensuring high separation efficiency of bulk charge. Therefore, two-dimensional semiconductor nanosheet materials exhibit unique and irreplaceable advantages in photocatalytic applications. As a new type of two-dimensional layered material, MXene has excellent physical and chemical properties, especially its surface is easily tunable with abundant terminal groups, unique morphological structure, superior optical absorption properties, and tunable band gap. And its superior electron transport efficiency endows MXene materials with unique optical, electrical, magnetic and thermal properties. In view of these unique properties of MXene materials, MXene materials should be very promising photocatalyst materials. Theoretical analysis found that the existence of surface -O terminal groups would greatly limit the adsorption capacity of N _on the surface of MXene material, which in turn affected its photocatalytic nitrogen fixation performance. In addition, the photocatalytic performance of MXene materials is limited by its interlayer stacking, complex surface modification functional groups, and limited number of photogenerated carriers, which seriously limit its photocatalytic nitrogen fixation performance.

Plasmonic Au nanoparticles have excellent optical properties and can excite their localized surface plasmon resonances through photon absorption, and are widely used in optical sensing, biomedicine, solar cells, and optical and electrocatalysis. Plasmonic Au nanoparticles have many advantages, including (1) the resonance wavelength can be systematically tunable in the ultraviolet to infrared light region, enhancing the light-harvesting ability; (2) generating high-energy hot carriers to drive chemical reactions; (3) The photothermal effect brought about by the conversion of light to heat during carrier decay has been widely used to drive photochemical reactions or enhance the photocatalytic performance of catalysts. Based on the many optical advantages of Au localized surface plasmon resonance, the construction of MXene/Au composites with interlayer interlayers can not only solve the limitations of MXene interlayer self-stacking, low utilization of interlayer active sites, etc. The plasmon resonance effect of Au enhances the photocatalytic nitrogen fixation effect of MXene materials.

SUMMARY OF THE INVENTION

The purpose of the present invention is to construct Ti ₃ C ₂ MXene composite photocatalyst, prepare Au nanospheres of different sizes and embed them between Ti ₃ C ₂ layers, which can not only solve the interlayer stacking problem of Ti ₃ C ₂ material itself, but also improve the _The utilization of interlayer active sites and the localized surface plasmon effect of Au nanospheres are used to enhance the photocatalytic nitrogen fixation effect of layered _Ti3C2 materials.

The invention also provides a method for preparing layered Ti ₃ C ₂ material, layered MXene material with local Ti reduction on the surface, and MXene/Au composite material with different Au sizes and different Au attachment positions, and the photocatalysis of the MXene/Au composite material Application of nitrogen fixation.

To achieve the above object, the technical scheme adopted in the present invention is:

A preparation method of MXene/Au photocatalytic nitrogen fixation material, the steps are as follows:

(1) Prepare layered Ti ₃ C ₂ material for use;

(2) Preparation of layered Ti ₃ C ₂ material with local Ti reduction on the surface: weigh the layered Ti ₃ C ₂ material and anneal it at 180-220 ℃ in a N ₂ /H ₂ mixed atmosphere with a hydrogen volume ratio of 5-10% After treatment for 5-7 hours, a layered Ti ₃ C ₂ material with local Ti reduction on the surface is obtained, which is denoted as r-Ti ₃ C ₂ ;

(3) The layered Ti ₃ C ₂ or r-Ti ₃ C ₂ material was uniformly dispersed in water by ultrasonic, and then the sodium citrate-coated Au sphere solution or the CTAB-coated gold sphere solution was added, and the ultrasonic dispersion was uniform, and rotary evaporation was carried out. Dissolve the solvent, dry, and get, every 0.1 g of layered Ti ₃ C ₂ or r-Ti ₃ C ₂ material requires 10 mL of water and 5 mL of sodium citrate-coated Au sphere solution or CTAB-coated gold sphere solution.

Further, the preparation process of the sodium citrate-coated gold spheres is as follows: 50 mL of chloroauric acid solution with a concentration of 0.01 wt % is stirred and heated to boiling; 2 mL to 4.5 mL of a sodium citrate solution with a concentration of 1 wt % is added to the In the boiling chloroauric acid solution, stirring and reacting for 20 min to 40 min, the solution of gold spheres coated with sodium citrate of different sizes is obtained.

Further, the addition amounts of 1wt% sodium citrate solution were 4.5mL, 3mL, and 2mL, respectively, to obtain sodium citrate-coated gold sphere solutions with sizes of 13nm, 16nm, and 20nm, respectively.

Further, the preparation process of the CTAB-coated gold spheres is as follows:

(1) Seed solution preparation: Add 0.25 mL of 0.01 M HAuCl ₄ solution to 9.75 mL of 0.1 M CTAB solution and mix well, then add 0.60 mL of newly prepared 0.01 M NaBH ₄ solution, shake well, and place at room temperature for 2~ 4h;

(2) Take 0.12 mL of seed solution and add it to a mixture of 9.75 mL, 0.1 M CTAB solution, 190 mL ultrapure water, 4 mL, 0.01 M HAuCl ₄ and 15 mL, 0.1 M ascorbic acid. Room temperature 8h ~ 12h.

Further, in step (2), the volume ratio of hydrogen in the N ₂ /H ₂ mixture is 5%, the annealing temperature is 200° C., the treatment time is 6h, and the gas flow rate is 50mL/min.

Further, the rotating speed during rotary evaporation was 80 r/min, the pressure was 0.1 MPa, and the heating temperature of the rotary evaporation flask was 50 °C.

Further, in step (3), the power during ultrasonic dispersion is 100W, and the drying is performed at 60°C.

The MXene/Au photocatalytic nitrogen fixation material prepared by the above preparation method.

Application of the above MXene/Au photocatalytic nitrogen fixation materials in photocatalytic nitrogen fixation.

More preferably, the specific process of photocatalytic nitrogen fixation is as follows: weigh 80 mg of photocatalytic nitrogen fixation material and put it into the photocatalytic reactor, add 50 mL of pure water, and uniformly disperse by ultrasonic, then, continue to pass N into the reactor with a flow rate of 100 mL/min. ₂ About 20min ~ 40min, then, turn on the 300 W xenon light source to illuminate the reaction solution in the photocatalytic reactor from above, adjust the nitrogen flow to 50 mL/min, and take 2 mL of the reaction solution at 1 hour intervals to separate the catalyst by centrifugation. Ammonia production was detected and calculated by chromogenic method.

The invention uses the technical means of rotary evaporation, combines the solvent kinetic driving method and the principle of surface charge repulsion/adsorption to construct a layered Ti ₃ C ₂ /Au composite material intercalated with Au spheres, and uses the localized surface of the Au spheres, etc. The photocatalytic nitrogen fixation efficiency of the composite material is finally improved. The method is also applicable to the construction of composite materials with sandwich structures of other layered materials and metal particles.

Description of drawings

Fig. 1 is the XRD patterns of Ti ₃ C ₂ , r-Ti ₃ C ₂ , Ti ₃ C ₂ /Au, r-Ti ₃ C ₂ /Au materials;

Fig. ₂ is the spectrum of Ti2p _of Ti3C2, r _{- Ti3C2} material;

Figure 3 is a scanning electron microscope image of Ti ₃ C ₂ and r-Ti ₃ C ₂ materials, wherein (a) Ti ₃ C ₂ , (b) r-Ti ₃ C ₂ ;

Figure 4 shows the SEM images and particle size distribution of Au spheres with different sizes, (a) Au coated with 13nm sodium citrate; (b) Au coated with 16nm sodium citrate; (c) 20nm sodium citrate coated Coated Au; (d) 20 nm CTAB-coated Au;

Fig. 5 is the SEM images of Ti ₃ C ₂ /Au and r-Ti ₃ C ₂ /Au materials with sandwich structure, (a) Ti ₃ C ₂ /Au, (b) r-Ti ₃ C ₂ /Au;

Figure 6 is a scanning electron microscope image of Ti ₃ C ₂ /Au and r-Ti ₃ C ₂ /Au materials attached to the layer edge, (a) Ti ₃ C ₂ /Au, (b) r-Ti ₃ C ₂ /Au;

Figure 7 shows the photocatalytic nitrogen fixation performance of Ti ₃ C ₂ , r-Ti ₃ C ₂ , Ti ₃ C ₂ /Au and r-Ti ₃ C ₂ /Au materials; (a) The ammonia production of different catalyst materials under the full spectrum ; (b) Ammonia production of different catalyst materials under visible light; (c) Comparison of ammonia production rates of different catalysts;

Figure 8 shows the comparison of the photocatalytic nitrogen fixation performance of the composites with different Au attachment positions.

Detailed ways

In order to make the technical purpose, technical solution and implementation effect of the present invention clearer, the technical solution of the present invention is further described, but the examples are intended to explain the present invention, and should not be construed as a limitation of the present invention. If the specific technology or condition is indicated, follow the technology or condition described in the literature in the field or according to the product specification.

Example 1

A preparation method of a Ti ₃ C ₂ /Au composite material with a sandwich structure, the process is as follows:

(1) Preparation of layered Ti ₃ C ₂ material: Weigh 2 g of Ti ₃ AlC ₂ ceramic phase precursor, add it to 20 mL of 40% HF solution, stir and etch at 35 °C for 48 h, and then centrifuge to obtain powder, And repeatedly rinsed with high-purity water to neutrality, the obtained powder was placed in a 60 ℃ vacuum oven to dry, to obtain a layered Ti ₃ C ₂ material.

(2) Preparation of layered Ti ₃ C ₂ material with local Ti reduction on the surface: Weigh 0.5 g of the layered Ti ₃ C ₂ material obtained above, in a N ₂ /H ₂ mixed atmosphere with a hydrogen volume ratio of 5% for 200 After annealing at ℃ for 6 hours, the gas flow rate was 50 mL/min, and the layered Ti ₃ C ₂ material with local Ti reduction on the surface (referred to as r-Ti ₃ C ₂ for short) was obtained. The spectrum of Ti 2p of Ti ₃ C ₂ and r-Ti ₃ C ₂ materials is shown in Figure 2. The main purpose of XPS spectrum analysis is to evaluate the existence form and relative content of Ti on the surface of Ti ₃ C ₂ and r-Ti ₃ C ₂ materials. It can be seen from Figure 2 that the changes in the relative intensities of the components of Ti ²⁺ , Ti ³⁺ , and Ti ⁴⁺ can be clearly observed in Ti ₃ C ₂ and r-Ti ₃ C ₂ materials. After thermal reduction treatment, the strength of Ti ²⁺ and Ti ³⁺ components in the r-Ti ₃ C ₂ material is significantly enhanced, while the strength of the Ti ⁴⁺ component content is significantly reduced, indicating that the Ti ⁴ on the surface of the Ti ₃ C ₂ material The ⁺ component is partially reduced to low-valence Ti ²⁺ and Ti ³⁺ components by thermal reduction treatment.

Figure 3 shows the SEM images of Ti ₃ C ₂ and r-Ti ₃ C ₂ materials. It can be seen from FIG. 3 that the prepared Ti ₃ C ₂ and r-Ti ₃ C ₂ materials are all multi-layer layered structures, and the process of thermal reduction treatment will not destroy their original layered structures.

(3) Preparation of Ti ₃ C ₂ /Au composites with sandwich structure: First, 0.1 g of Ti ₃ C ₂ material was weighed, and it was uniformly dispersed in 10 mL of high-purity water by ultrasonic (100W), and then 5 mL of a certain particle concentration was added The solution of Au spheres coated with sodium citrate of different sizes was uniformly dispersed by 100W ultrasonic wave for 10min. With the help of rotary evaporation technology, the rotation speed was 80 r/min, the pressure was 0.1MPa, and the heating temperature of the water bath to the rotary evaporation flask was 50 ℃, the water solvent is continuously evaporated. At the same time, because the surface of the gold spheres coated with sodium citrate is negatively charged, and the edges of the Ti ₃ C ₂ layer are also negatively charged, there is electrostatic repulsion. Therefore, under the synergistic effect of the kinetic driving effect of the water solvent and the electrostatic repulsion effect of the water solvent during the rotary evaporation process, the Au sphere particles are gradually driven to the interlayer of the layered _Ti3C2 to construct a _{sandwich -} structured _Ti3C2 / The Au composite material was dried in a vacuum oven at 60 °C.

(4) Preparation of r-Ti ₃ C ₂ /Au composite material with sandwich structure: replace the Ti ₃ C ₂ material with 0.1 g of r-Ti ₃ C ₂ , and other steps are the same as step (3).

Wherein, the preparation process of sodium citrate coated gold ball is as follows:

50mL, 0.01wt% chloroauric acid solution was heated and stirred to boiling (about 150 ℃); 4.5mL, 3mL, 2mL were added to the boiling chloroauric acid solution with a concentration of 1wt% sodium citrate solution, and the reaction was stirred for 30min, That is, 13nm, 16nm, 20nm sodium citrate-coated Au spheres were obtained. The SEM images and particle size distributions of Au spheres with different sizes are shown in Figure 4(a~c), in which (a) Au coated with 13nm sodium citrate; (b) Au coated with 16nm sodium citrate; ( c) 20 nm sodium citrate-coated Au.

Fig. 1 shows the XRD analysis of Ti ₃ C ₂ , r-Ti ₃ C ₂ , Ti ₃ C ₂ /Au, r-Ti ₃ C ₂ /Au materials. It can be seen from Fig. 1(a) that after etching by HF acid, the The diffraction peaks at 2θ=8.9, 18.4 and 27.5 ^o correspond to the (002), (004) and (008) crystal planes, respectively. The diffraction peaks originally belonging to Ti ₃ AlC ₂ are obviously reduced or disappeared, indicating that Ti ₃ AlC ₂ After reacting with HF acid, the original structure was destroyed, and the Al layer was etched and peeled off, indicating that the layered Ti ₃ C ₂ material was successfully prepared. It can be seen from Figure 1(b) that, compared with the Ti ₃ C ₂ material, the r-Ti ₃ C ₂ material after thermal reduction treatment has a new diffraction peak at 23.3 ^o , which corresponds to the (006) crystal plane, indicating that the After thermal reduction treatment, the interlayer spacing is enlarged, and the interlayer structure is better. It can be seen from Fig. 1(c) that the characteristic diffraction peaks of cubic Au are clearly displayed in the XRD patterns of Ti ₃ C ₂ /Au and r-Ti ₃ C ₂ /Au composites after being compounded with Au spherical nanoparticles. Indicates the successful compounding of the Au sphere particles.

FIG. 5 is a scanning electron microscope image of Ti ₃ C ₂ /Au, r-Ti ₃ C ₂ /Au materials prepared from Au coated with 13 nm sodium citrate. It can be seen from Fig. 5 that the prepared Ti ₃ C ₂ /Au and r-Ti ₃ C ₂ /Au materials present a clear sandwich structure, and the Au nanospheres are uniformly embedded in the Ti ₃ C ₂ , r-Ti ₃ C ₂ layers of material.

Example 2

A preparation method of Ti ₃ C ₂ /Au, r-Ti ₃ C ₂ /Au composite material with layer edges attached, the process is as follows:

Au spheres coated with 5 mL of cetyltrimethylammonium bromide (CTAB) were used to replace the solution of Au spheres coated with sodium citrate, and the others were the same as in Example 1.

The preparation process of CTAB-coated gold spheres is as follows:

Seed solution preparation: Add HAuCl ₄ solution (0.01 M, 0.25 mL) to CTAB solution (0.1 M, 9.75 mL) and mix well, then quickly add freshly prepared NaBH ₄ solution (0.01 M, 0.60 mL), shake well, room temperature placed for 3h.

Add 0.12 mL of seed solution to a mixture of CTAB (0.1 M, 9.75 mL), ultrapure water (190 mL), HAuCl ₄ (0.01 M, 4 mL) and ascorbic acid (0.1 M, 15 mL), shake well , placed at room temperature for 10h, the scanning electron microscope image and particle size distribution are shown in Figure 4d. From Figure 4d, it can be seen that Au coated with 20nm CTAB is obtained.

FIG. 6 is a scanning electron microscope image of Ti ₃ C ₂ /Au and r-Ti ₃ C ₂ /Au materials attached to the layer edge. As shown in the figure, the Au nanospheres are uniformly attached to the layer edges of Ti ₃ C ₂ and r-Ti ₃ C ₂ materials.

Photocatalytic nitrogen fixation test

The photocatalytic nitrogen fixation reaction test was specifically completed in a sealed quartz photocatalytic reactor with a diameter of 6 cm and a volume of 100 mL at room temperature. The specific experimental process is as follows: Weigh 80 mg of the catalyst into the photocatalytic reactor, add 50 mL of pure water, and ultrasonically disperse for 5 minutes to make the catalyst powder evenly dispersed. Then, N ₂ was continuously passed into the reactor at a flow rate of 100 mL min ⁻¹ for about 30 min, and the reactor was in a state of continuous gas circulation at this time. Subsequently, a 300 W xenon lamp light source was turned on to illuminate the reaction solution in the photocatalytic reactor from above, and the nitrogen flow was adjusted to 50 mL/min. Every 1 hour, 2 mL of the reaction solution was taken and centrifuged to separate the catalyst, and the ammonia production was detected and calculated by the Nessler reagent color development method.

Fig. 7 is Ti ₃ C ₂ /Au prepared by Ti ₃ C ₂ , r-Ti ₃ C ₂ , Ti ₃ C ₂ /Au, r-Ti ₃ C ₂ /Au (13nm sodium citrate coated Au, r -Ti ₃ C ₂ /Au) material photocatalytic nitrogen fixation performance map. It can be seen from Figure 7 that the Ti ₃ C ₂ material has almost no photocatalytic nitrogen fixation effect. After thermal reduction treatment, the photocatalytic nitrogen fixation performance of r-Ti ₃ C ₂ is improved, and its ammonia production rate is 1.25 μmol/h/h under white light. g _cat. , indicating that the r-Ti ₃ C ₂ material presents more effective active sites after reduction treatment. After compounding with Au spheres, the ammonia production rate of Ti ₃ C ₂ /Au under white light is 1.91 μmol/h/g _cat. , and the improvement of its photocatalytic nitrogen fixation effect should mainly come from the thermionic effect of Au spheres. However, the photocatalytic nitrogen fixation efficiency of r-Ti ₃ C ₂ /Au material is significantly improved, and its ammonia production rate reaches 22.6 μmol/h/g _cat. , which proves that the localized surface plasmon effect of Au helps to improve the The utilization rate of light energy brings more effective high-energy hot carriers, thereby improving the photocatalytic nitrogen fixation efficiency of the composite material.

Figure 8 is a comparison of the photocatalytic nitrogen fixation performance of the 20nm sandwich structure r-Ti ₃ C ₂ /Au prepared in Example 1 and the 20 nm layer-attached r-Ti ₃ C ₂ /Au composite prepared in Example 2 . The results show that the ammonia production rate of r-Ti ₃ C ₂ /Au@CTAB with the same Au size and the same Au loading at the layer edge position is 16.63 μmol/h/g _cat. , which is significantly lower than that of the interlayer embedded sandwich structure The ammonia production rate of r-Ti ₃ C ₂ /Au@citrate is 26.82 μmol/h/g _cat. , which indicates that the layer-edge attachment reduces the utilization of interlayer active sites and leads to the reduction of its photocatalytic nitrogen fixation performance.

Finally, it should be noted that, in the present invention, the parameters related to the construction of the composite material can be adjusted within a corresponding range, and the apparent size of Au spheres, the composite concentration of Au spheres, and the type of MXene materials can be replaced or modified accordingly. . The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described with reference to the preferred embodiments of the present invention, those of ordinary skill in the art should Various changes may be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (10)

1. a preparation method of MXene/Au photocatalytic nitrogen-fixing material, is characterized in that, step is as follows: (1) Prepare layered Ti ₃ C ₂ material for use; (2) Preparation of layered Ti ₃ C ₂ material with local Ti reduction on the surface: weigh the layered Ti ₃ C ₂ material and anneal it at 180-220 ℃ in a N ₂ /H ₂ mixed atmosphere with a hydrogen volume ratio of 5-10% After treatment for 5-7 hours, a layered Ti ₃ C ₂ material with local Ti reduction on the surface is obtained, which is denoted as r-Ti ₃ C ₂ ; (3) Disperse the r-Ti ₃ C ₂ material uniformly in water by ultrasonic, then add sodium citrate-coated Au sphere solution or CTAB-coated gold sphere solution, ultrasonically disperse evenly, spin out the solvent, and dry to obtain , each 0.1 g r-Ti ₃ C ₂ material requires 10 mL of water and 5 mL of sodium citrate-coated Au sphere solution or CTAB-coated gold sphere solution. 2. the preparation method of MXene/Au photocatalytic nitrogen-fixing material according to claim 1, is characterized in that, the preparation process of the gold sphere covered by sodium citrate is as follows: 50mL, concentration is that the chloroauric acid solution of 0.01wt% is stirred Heating to boiling; respectively adding 2mL~4.5mL sodium citrate solution with a concentration of 1wt% into the boiling chloroauric acid solution, stirring and reacting for 20min~40min, to obtain gold sphere solutions coated with sodium citrate of different sizes . 3. the preparation method of MXene/Au photocatalytic nitrogen-fixing material according to claim 2, is characterized in that, the addition of 1wt% sodium citrate solution is respectively 4.5 mL, 3 mL, 2 mL, and the obtained size is 13 nm, 2 mL, respectively. 16 nm, 20 nm sodium citrate coated gold sphere solution. 4. the preparation method of MXene/Au photocatalytic nitrogen fixation material according to claim 1, is characterized in that, the preparation process of the gold ball of CTAB coating is as follows: (1) Seed solution preparation: Add 0.25 mL of 0.01 M HAuCl ₄ solution to 9.75 mL of 0.1 M CTAB solution and mix well, then add 0.60 mL of newly prepared 0.01 M NaBH ₄ solution, shake well, and place at room temperature for 2~ 4h; (2) Take 0.12 mL of seed solution and add it to a mixture consisting of 9.75 mL, 0.1 M CTAB solution, 190 mL ultrapure water, 4 mL, 0.01 M HAuCl ₄ and 15 mL, 0.1 M ascorbic acid, shake well, Room temperature 8h ~ 12h. 5. The preparation method of the MXene/Au photocatalytic nitrogen-fixing material according to claim 1, wherein in step (2), the volume ratio of hydrogen in the N ₂ /H ₂ mixed gas is 5%, and the annealing temperature is 200°C, The treatment time was 6 h, and the gas flow was 50 mL/min. 6. The preparation method of the MXene/Au photocatalytic nitrogen-fixing material according to claim 1, characterized in that, the rotating speed during rotary evaporation is 80 r/min, the pressure is 0.1 MPa, and the heating temperature of the rotary evaporation flask is 50 °C. 7 . The preparation method of MXene/Au photocatalytic nitrogen fixation material according to claim 1 , wherein, in step (3), the power during ultrasonic dispersion is 100W, and the drying is performed under vacuum at 60° C. 8 . 8. The MXene/Au photocatalytic nitrogen-fixing material prepared by the preparation method of any one of claims 1 to 7. 9. Application of the MXene/Au photocatalytic nitrogen fixation material of claim 8 in photocatalytic nitrogen fixation. 10. application according to claim 9, is characterized in that, process is as follows: take 80mg photocatalytic nitrogen fixation material and put into photocatalytic reactor, add 50 mL pure water, ultrasonically disperse uniformly, then, with 100 mL/min The flow rate of N ₂ was continuously passed into the reactor for 20 min ~ 40 min, and then, the 300 W xenon lamp light source was turned on to illuminate the reaction solution in the photocatalytic reactor from above, and the nitrogen flow rate was adjusted to 50 mL/min. Every 1 hour, take 2 The reaction solution was centrifuged to separate the catalyst, and the ammonia production was detected and calculated by the Nessler reagent color development method.

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